1. Field of the Invention
This invention relates, in general, to laser printers and, more specifically, to registration enhancing systems for hard copy scanning printers, including laser printers.
2. Description of the Prior Art
In some applications, the location of the printed image upon the hard copy or sheet material on which the printed image is produced is not critical to acceptable operation of the printer. Generally speaking, precise printers, such as laser printers, which are used to produce a single copy of an image, perform satisfactorily if the image is registered on the hard copy paper within a fraction of an inch of the true registration position. However, with more and more printers using images stored in digital memory to construct the image on the hard copy material, more precise control of the location of the image is possible than with optical-type duplicators and copiers.
Registration is particularly important when the printer is being used in some fashion for color printing or reproduction. In many color printing systems, several colors are printed separately on top of each other to form a composite color image. In order to have a good quality color image, it is necessary that the individual colors be aligned or registered very accurately on the printed paper. Color registration which is off just a very small fraction of an inch will produce color images that are not acceptable to present standards. Precise registration is also desired for machines which make masters to be used in color printing operations. With the resolution of such machines being greater, in some cases, than 1,000 lines/inch, the opportunity exists for producing very sharp composite color images if the registration between the different color masters is properly maintained.
A lack of registration in laser printers occurs for a variety of reasons. One of the reasons involves synchronization of the laser scanning beam with the surface onto which the beam is focused. One type of currently available laser printer uses a photoconductive web material constructed in an endless belt configuration which rotates around rollers to move the latent image created by the laser beam through the other stations of the printer to eventually produce the hard copy output. With this type of printing system, the scanning of the laser beam must be coordinated with the speed and the position of the drum and with the electronic control of the modulation, or illumination control, of the laser beam. Typically, such systems include a means for scanning the laser beam linearly across the web surface to form one line of the image at a time. Successful line scans are used to construct an entire image on the web. Normally, the web is moving to provide a displacement from line to line, and the laser beam moves from one end of the line to the other end of the line.
Many printers operate with the location of the beam and the location of the laser beam on the scan line, not always synchronized to each other. In other words, when the web is in position to receive the image to be constructed by the laser beam, the laser beam may be already oriented to scan somewhere within the line. Since the scan is always started at the beginning of the line, a delay is necessary before the scan is commenced. The amount of delay depends upon the position of the laser beam within the line when the images to be started. Starting of the image does not occur until the laser beam returns to the edge of the web and trips or is detected by a photodetector which signifies that the laser beam is in a position to start accepting image information which will be written or imaged onto the photoconductive web. As a result, it is possible, at a worst case analysis, that the laser beam will just have passed the edge detector when it is to start writing an image on the web. Since the image does not start writing until the laser beam scans the entire line, it starts again at the edge, significant misregistration on the web can occur. In this case, the image written onto the web can be misregistered by an amount equal to the width of one scan line. When the printer is used to make masters for multicolor printing processes and to make color prints, this type of misalignment in the registration between the various color masters or the print layers produces undesirable copies, or prints.
Even monochrome reproductions that are not intended for use in composite images may require accurate reproduction of finely-spaced lines or other minute, repetitive image components. Such components in a detailed original (for example, a map or chart) must be reproduced with fidelity to the original image. Non-uniformity in the writing of the latent image on the media can cause a noticeable image degradation: a finely-ruled rectangular grid in a reproduction of an image will appear uneven in thickness and spacing; a reproduction of an image having a gray-scale pattern will have noticeable density grating and variations as well.
In a web-base reproduction or printing apparatus, the web is typically supported by several free turning rollers and driven by one drive roller. This roller, in turn, is driven by a motor. (An alternative reproduction apparatus, a driven drum assembly, is substituted for the web and rollers.) Because these driving assemblies are electro-mechanical systems, there is a tendency for the web (or drum) to vary in speed as it is driven. Typically, a low-cost synchronous motor will vary in speed as the line frequency varies. Moreover, because the web or drum is photosensitive, it typically is exposed line by line by a laser beam or linear LED array, any speed variations of the media will cause the exposure lines to be written at different inter-line spacing. The sum of this effect is that different image frames on the photosensitive medium will have images of different length. Those image frames, when developed and transferred, will produce reproductions with correspondingly disparate frame length.
In the production of a latent image frame, any speed variation of the photosensitive medium must be held to a minimum; otherwise the success of latent image frames are likely to have different lengths. The resulting misregistration of the developed and transferred image frames must be held within acceptable limits. Most variations in web speed are caused by a variety of mechanical factors which effect the web transport speed, such as roller bearing or drive motor friction, line voltage changes, line frequency changes, asymmetry of the drive motor poles or misalignment of the apparatus support chassis with respect to the axis of the web transport rollers and the drive means. Much improvement has been made in the an to reduce such gross speed variations to a level that is acceptable for most printing and reproduction applications. However, as a motor is controlled more and more accurately, the cost of this exacting motor speed control can become prohibitive. As an alternative, a low cost synchronous motor can be used to drive the photoconductor and compensation in the scanning of the laser be made to compensate for speed variations in the photoconductor.
In the past to accomplish low flutter, operator D.C. servo motors were used. Not only were the motors themselves expensive, but they required costly control systems. They also required very clean, high current and low voltage power supplies to operate properly. Not only was this expensive, but was large and bulky.
In Mager et al U.S. Pat. No. 4,835,545, a photosensitive media moving in a first direction, relative to a laser light beam scanning in a second direction, incurs velocity variation which cause variations in the absolute and relative heights of white and black image features. An instantaneous velocity error calculation is used to adjust the intensity of the laser light beam to be proportionately brighter (dimmer), exposing a wider (narrower) scan line, on a faster-moving (slower-moving) media region.
In Hoshino et al U.S. Pat. No. 4,803,515, an image-forming apparatus includes movable image-bearing member and a driver for driving the image-bearing member. The time interval required for the image-bearing member to move from a latent image-forming position is an integer multiple of a period of the drive non-uniformity inherent in the driver.
In Lama et al U.S. Pat. No. 4,801,978, a control circuit is provided in an electronic printer utilizing an image write bar to compensate for the effects of vibration in a rotating photoconductive member.
In Kramer et al U.S. Pat. No. 4,785,325, a document imaging system incorporates a mechanism for adjusting the speed ratio between the document scanning system and the photoreceptor.
In Ritter et al U.S. Pat. No. 4,779,944, an integrated laser scanning system for scanning a modulated beam across an image surface (receptor) is provided. The variation in rotational speed of the receptor is discussed as a major source of scan line spacing error in images recorded with the laser scanner.
In Hanlan U.S. Pat. No. 4,361,260, a register control is provided for a web handling apparatus wherein a control provides a time and modification to a sensed speed signal for providing a modified speed command signal when the system is out of registration.
In Draugelis et al U.S. Pat. No. 4,082,443, digital logic circuitry insures that latent images are correctly placed on the photoconductor, by varying the time of the flash assembly.
In Kushner U.S. Pat. No. 3,934,505, a method and apparatus are disclosed wherein a signal proportional to the speed of a moving web is compared with a signal proportional to the speed of a motor-driven rotary printing member, and a resultant corrective signal is transmitted to the controller for the motor, the linear speed of the rotary printing member is thus made equal to the web speed.
Young et al U.S. Pat. No. 4,972,208 teaches a method and apparatus for image frame length control in a scanning apparatus employing a driven photosensitive media having a median thickness and variations therefrom. The method comprises the steps of storing a profile with thickness variations of a photosensitive media, receiving image information, rotating the media to provide a location for creation thereon of a latent image frame according to the received image information, calculating the predicted frame length of the latent image according to the thickness profile and the image frame location, offsetting an imagewise exposure of the media by an interval proportional to one-half of the disparity between the predicted image frame length and the correct image frame length, and imagewise exposing the media on a line-by-line basis according to the stored image information to create a latent image thereon. The latent image frame length is thus corrected with respect to the predicted frame length.
Jamzadeh U.S. Pat. No. 4,893,135 discloses an apparatus and method to enhance the registration of laser printers using endless web-type or drum photoconductive surfaces. The enhancement is provided by delaying the feeding of paper into the image transfer station by the amount of time the latent image is delayed in scanning at the exposure station due to the position of the laser beam when scanning was to begin. A start-of-page (SOP) signal is provided by the location of the web and indicates that the web is in a position to accept laser scanned images thereon. Circuitry is included to determine the time difference between the receipt of the start-of-page signal and the receipt of the start-of-line (SOL) signal which is activated by the laser beam when it returns to the edge of the web to begin starting the scan of a new line. This delay can be as much as one full line of the laser beam as just started scanning a line at the time the start-of-page signal was issued. The delays between the two signals are convened to counts which are then converted to time delays that are used to extend or delay the conventional signal issued by the master system controller to feed paper into the transfer region.